Environmental factors and endogenous metabolic processes create polymerase-blocking DNA lesions, which can stall DNA replication machinery and cause genome instability. To avoid such events, cells have evolved a mechanism called translesion synthesis (TLS), which uses low fidelity polymerases to allow DNA synthesis across the lesions without actual repair. Our long-term goal is to understand regulatory mechanisms of TLS and determine its importance in prevention of genome instability and tumor formation. Recently, our group and others identified Spartan (also known as DVC1) as a novel regulator of TLS. We demonstrated that Spartan depletion results in accumulation of the error-prone TLS polymerase Pol zeta and higher UV-induced mutagenesis. Building on these observations, we now hypothesize that Spartan negatively regulates Pol zeta complex, and thereby prevents genome instability and tumor formation. To gain more insights into the mechanism of TLS regulation by Spartan and its physiological relevance, we propose to: (1) Study the mechanism of Pol zeta regulation by Spartan, (2) Investigate the cause of DNA damage and cell-cycle defects in Spartan knockout cells, (3) Examine the role of Spartan in genome stability and tumor suppression in mouse. Aim 1 will focus on the SprT domain, a putative metalloprotease domain of Spartan, and determine its role in Pol zeta regulation by characterizing its activity in vitro. n addition, we will investigate how Rev1, a regulator of Pol zeta, is involved in the complex formation of Pol zeta. In Aim 2, we will investigate the cause of DNA damage and cell cycle defects in Spartan knockout cells and determine whether it is related to Spartan's function in TLS. Finally in Aim 3, we will determine whether Spartan is important to prevent mutations, genome instability and tumor formation using mouse models. In summary, these studies will not only shed new light on the regulation of TLS by Spartan, but also provide novel insights into the role of TLS regulation in genome maintenance and tumor suppression.